Conventionally, for the purpose of increasing the wear resistance of a tool which is made of high-speed tool steel, cemented carbide, cermet or CBN sintered body, the tool is covered with a hard coating formed of a nitride, carbide or carbon-nitride of Ti, Cr, Hf or Zr, by a physical vapor deposition method (PVD method), a chemical vapor deposition method (CVD method), or other suitable method, such that the hard coating has the average thickness of 0.50-10.0 .mu.m. In recent years, for the purpose of increasing the rust or oxidation resistance of the hard coating so as to satisfy the requirement for increased cutting velocity, there has been widely used a tool which is formed by covering the substrate or base material made of a cemented carbide with a hard coating of AlTiN or AlTiCN by the PVD method. For example, JP-B2-4-53642 discloses a hard coating which has a thickness of 0.50-10.0 .mu.m and which is formed of a composite solid solution of carbide, nitride or carbon nitride of Al and Ti. Further, JP-B2-5-67705 discloses a hard coating which has a thickness of 0.80-10.0 .mu.m and which is formed of a wear resistant coating having a chemical composition wherein the content of aluminum is in a range of 56-75%, in an attempt to further increase the heat resistance of the hard coating. Still further, JP-A-7-97679 discloses a hard multilayer coating which has a total thickness of 0.50-10.0 .mu.m and a stoichiometrically aluminum-rich bulk composition, wherein titanium-rich ultra thin AlTiN layers and aluminum-rich ultra thin AlTiN layers are alternately superposed on each other such that the pitch or spacing interval between the adjacent tanium-rich ultra thin AlTiN layers, or between the adjacent aluminum-rich ultra thin AlTiN layers is in a range of 0.50-20.0 nm (nanometer).
In recent years, the required cutting velocity has been further increased while the work material to be cut has become harder. Where the conventional coated tool is used for cutting the harder work material at a required high cutting velocity, the coating of the tool tends to be slightly broken or chipped instead of being worn. Particularly, the aluminum-rich AlTiN coating is easily chipped due to its high hardness of 2600 Hv or more. Thus, the conventional coated tool does not exhibit the desired degree of wear resistance, irrespective of whether the tool is covered by a coating constituted by a single layer, a coating constituted by a plurality of layers, or a coating constituted by layers each having a considerably small thickness. The coating of the tool tends to be chipped more easily when the hardness of the coating is relatively high, so that the tool with the hard coating is not capable of exhibiting an expected or satisfactory degree of wear resistance.